Solved.

We are paid 4,00 EUR now.

There could be two reasons why we got paid 3,00 per hour:

1. either it was some kind of a mistake

2. or Mark probed us: will they complain or not? ;)

The second possibility is quite an intrigue, what they call it: "conspiracy theory" :). But I don't have a problem with that; it would be quite intelligent to try this kind of thing on us!
;)